1. Field of the Invention
The present invention relates to a method of inspecting holes, such as contact holes or via holes, using a charged-particle beam, the holes being forming during a manufacturing process for semiconductor devices, such as ICs and LSIs, to understand the state of formed holes, especially the state of etched holes.
2. Description of the Related Art
A semiconductor device consists, for example, of a silicon wafer (silicon substrate) on which a multilayer structure is formed. In this multilayer structure, a dielectric layer is formed between certain layers. Contact holes or via holes are formed in this dielectric layer. These contact holes or via holes are filled by metallization (with conductive material) to make electrical connections between the certain layers. In the following description, contact holes are taken as an example.
Such contact holes are formed by applying resist to the dielectric layer, exposing the resist according to the pattern of the contact holes, and then performing development and etching steps during a process for fabricating a semiconductor device.
Where these contact holes are formed, if a contact hole Ca extends even somewhat into a conductive layer D through a dielectric layer A (known as overetching) as shown in FIG. 1(a), or if a contact hole Cb is formed while leaving behind a part of the dielectric layer A (known as underetching), the final product does not function normally as a semiconductor device and is a defective device.
Therefore, inspecting the state of the contact holes after they have been formed is important in determining whether the subsequent process sequence is to be carried out or not. Furthermore, it can be judged according to the results of this inspection (i.e., the state of the formed contact holes) whether the development or etching that is a previous step was appropriate or not. Additionally, the process for forming the contact holes can be analyzed for causes of defects.
The state of contact holes formed in this way is inspected nondestructively, for example, by electron beam irradiation from a scanning electron microscope (SEM). In particular, contact holes are scanned with an electron beam. A secondary electron image of the contact holes is displayed on the viewing screen of a display unit, based on detected secondary electrons. This image is observed. Thus, the state of the etched contact holes (i.e., the etching process) is inspected (see, for example, Great Britain Patent No. 2338297A).
In recent years, elements forming semiconductor devices have become decreased in size. Also, these elements have tended to be formed in plural layers. With this trend, contact hole diameters have decreased. Also, their depths have increased. Consequently, contact holes have increased in aspect ratio (depth/diameter). Therefore, the efficiency at which secondary electrons from inside holes are captured has decreased greatly. This has made it difficult to precisely determine the state of formed contact holes, i.e., the etching process.
In some cases, a wafer is cut along a plane including the center axis of a contact hole, and the cross section is observed with an SEM. In recent years, however, larger wafer sizes (e.g., having diameters of 200 to 300 mm) have been used. Therefore, it is difficult to create cross sections adapted for observation with an SEM. Furthermore, the aspect ratio (depth/diameter) of contact holes has tended to increase as mentioned previously, thus making it difficult to perform the cutting operation itself for cutting a wafer along a plane including the center axis of a contact hole.
A method of evaluating the thickness of a film remaining on the bottom surface of a contact hole has been recently proposed. An electron beam is made to hit this contact hole having the remaining film on its bottom surface. The value of an electric current flowing through the remaining film into a support substrate located immediately under the opening is measured. A pseudo-remaining film on the bottom surface of the opening is assumed. A reference sample is used to previously create a comparison table that correlates the aforementioned current value with the thickness of the pseudo-remaining film. Then, an electron beam is directed to a contact hole to be inspected. The value of the current passing through the film is measured. The thickness of the film remaining on the bottom surface of the contact hole is evaluated based on the measured current value by referring to the comparison table. This method is only capable of evaluating the thickness of the film remaining on the bottom surface of the contact hole.
Where contact holes are formed during a process for fabricating semiconductor devices, underetching may occur, leaving behind a film in contact holes. In practice, overetching may also take place. That is, contact holes extend into the substrate. In the latter case, it is necessary to determine the depth of the contact holes into the substrate (i.e., etch depth of the contact holes into the substrate) based on results of an inspection of the contact holes.
It is an object of the present invention to provide a novel method of inspecting a contact hole or holes using a charged-particle beam in such a way that the state of the contact holes can be grasped (i.e., it is possible to know how the holes are etched).
A method of inspecting a hole using a charged-particle beam in according with the present invention comprises the steps of: irradiating the hole with the charged-particle beam, the hole being formed in an etched layer on a substrate forming a sample to be inspected; detecting an electric current flowing between the inspected sample and ground as a result of the irradiation; and finding the etch depth of the hole into the substrate regarding the inspected sample, based on a relation of the current flowing between a reference sample and ground to the etch depth of the hole into the substrate, the relation being previously found using the reference sample.
Another method of inspecting a hole using a charged-particle beam in accordance with the present invention comprises the steps of: irradiating the hole with the charged-particle beam, the hole being formed in an etched layer on a substrate forming a sample to be inspected; detecting an electric current flowing between the inspected sample and ground as a result of the irradiation; and finding how the hole in the inspected sample is etched, based on a relation of the current flowing between a reference sample and ground to etch depths of holes into the substrate and on a relation of the current to remaining film thickness in the holes, the relations being previously found using the reference sample.
A further method of inspecting a hole using a charged-particle beam in accordance with the present invention comprises the steps of: irradiating the hole with the charged-particle beam, the hole being formed in an etched layer on a substrate forming a sample to be inspected; detecting an electric current flowing between the inspected sample and ground as a result of the irradiation; comparing the detected current with a reference electric current previously found from a precisely etched reference sample, the reference electric current flowing between the reference sample and ground; and judging that the hole in the inspected sample has been precisely etched if the two compared currents are the same, that the hole has been overetched if the former current is greater than the latter current, and that the hole has been underetched if the former current is smaller than the latter current.
Yet another method of inspecting holes using a charged-particle beam in accordance with the present invention comprises the steps of: irradiating a region containing the holes with the charged-particle beam, the holes being formed in an etched layer on a substrate forming a sample to be inspected; detecting an electric current flowing between the inspected sample and ground as a result of the irradiation; repeating these steps for plural regions previously established on the inspected sample; obtaining data about the distribution of etch depths of holes in the inspected sample into the substrate, based on the detected current and on a relation of a reference current to etch depths of holes into the substrate, the reference current flowing between a reference sample and ground, the relation being previously found using the reference sample; and displaying a map based on the obtained data about the distribution on a display unit.
Still another method of inspecting holes using a charged-particle beam comprises the steps of: irradiating a region containing the holes with the charged-particle beam, the holes being formed in an etched layer on a substrate forming a sample to be inspected; detecting an electric current flowing between the inspected sample and ground as a result of the irradiation; repeating these steps for plural regions previously established on the inspected sample; obtaining data about the distribution of etch depths of the holes in the inspected sample into the substrate, based on a previously found relation of electric current flowing between the reference sample and ground to etch depths of holes into the substrate and on a previously found relation of the electric current to remaining film thickness in the holes; and displaying a map on a display unit, based on data obtained about the distribution of degrees of etching in the holes in the inspected sample.
An additional method of inspecting holes using a charged-particle beam in accordance with the present invention comprises the steps of: preparing a reference sample precisely etched; preparing an unknown sample that is not known whether it has been etched precisely or not; irradiating a region of the reference sample containing holes with the charged-particle beam; detecting an electric current flowing between the reference sample and ground; repeating these irradiating and detecting steps for previously established plural regions on the reference sample; irradiating a region of the known sample containing holes with the charged-particle beam; detecting an electric current flowing between the unknown sample and ground; repeating these irradiating and detecting steps for previously established plural regions on the unknown sample; finding data about current distributions on the reference and unknown samples; creating graphs indicative of the characteristics of the currents flowing through the regions of the reference sample and unknown sample; and displaying the graphs side by side on a display unit.
Other objects and features of the invention will appear in the course of the description thereof, which follows.